Tailored Materials for Surface-Enhanced Raman Spectroscopy
Surface-enhanced Raman spectroscopy (SERS) is a useful analytical tool that has grown in popularity since its discovery 40 years ago. Presently, a large number of materials are used as SERS substrates, including thiol-functionalised nanoparticles which form a large class of SERS-active materials as they are able to form selfassembled monolayers on the surface of a substrate and provide surface properties that may be tailored by altering the adsorbed thiol. Mixtures of thiols have been used to provide a wider range of surface properties but there is considerable uncertainty surrounding the mechanism of thiol adsorption, and so the aim of this project has been to develop an understanding of the kinetics and mechanism of the thiol adsorption process, specifically the co-adsorption of pairs of thiols onto silver nanoparticles. Normally the chosen thiols are added to the nanoparticle suspension followed by an aggregating agent which will create coupled surface plasmons leading to an enhanced signal. However, any changes in the SERS signal with time will be due to a combination of increasing population of thiol at the surface and the enhancement factor of the particle aggregates as aggregation progresses. It was therefore necessary to find conditions under which the aggregates would provide a stable enhancement factor and although a large number of colloid/salt combinations produced rapid aggregation followed by precipitation and subsequent loss of SERS signal, it was found that by using low concentrations of MgSO4, reasonably strong SERS enhancing aggregates which gave stable signals over 10 minutes could be generated. Once this method of preparing stable pre-aggregated colloids had been established, the adsorption of single thiols was monitored over time using SERS. Although using pre-aggregation generally reduced the signal levels making it difficult to monitor thiols with low scattering cross-sections, such as aliphatic thiols, it was still possible to use the batch system to measure the surface composition of a number of pairs of thiols with different initial feedstock solution compositions. A number of the pairs investigated were found to produce behaviour where the surface was dominated by one thiol when equal amounts of both thiols were added to the nanoparticles. With the batch system, the rate of adsorption was typically faster than the timescale of the measurements so the final stage of the research was the development of a flow iii system that could be used in conjunction with SERS to monitor the kinetics of thiol adsorption at faster timescales. A robust, reliable system was developed using FEP tubing and connectors, which allowed thiol adsorption to be monitored at times as early as 0.14 seconds. The adsorption of both aromatic and aliphatic thiols was shown to be rapid, with 50 % surface coverage reached within 1 second for the former and within 5 seconds for the latter. The flow system also allowed for the analysis of mixtures of thiols at early timescales and the majority of the results showed that the surface composition was established from the earliest time point and thus determined from the initial adsorption of the thiols. The only thiol pair tested where this was not the case was a mixture of a cysteamine hydrochloride (CYS) and mercaptobenzoic acid (MBA), which showed a surface composition that initially reflected the feedstock before the composition started to change into a more equal ratio as the reaction proceeded to completion
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